1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of transmitting a reference signal in a multiple antenna system.
2. Related Art
To maximize performance and communication capability of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention in recent years. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. In this case, the weight can be expressed by a weight vector or a weight matrix, which is respectively referred to as a precoding vector or a precoding matrix.
At present, it is considered in a current multiple antenna system that a base station (BS) uses a plurality of power amplifiers to perform multiple antenna transmission whereas a user equipment (UE) uses one power amplifier to perform multiple antenna transmission. The UE using one power amplifier may obtain a receive diversity effect by receiving an Rx signal through multiple antennas. However, since the UE uses a single antenna when transmitting a signal, the UE cannot obtain a multiple antenna effect such as transmit diversity, spatial multiplexing, or the like. To increase the multiple antenna effect in transmission, an antenna selection scheme is used. The antenna selection scheme includes a closed-loop antenna selection scheme in which an antenna to be used is reported to a transmitting side designated by a receiving side and an open-loop antenna selection scheme in which the antenna to be used is randomly selected by the transmitting side. A power amplifier of the UE has a maximum output (e.g., the maximum output of the UE may be 23 dBm), and uplink power control is performed according to the maximum output.
To improve an uplink transfer rate, a multiple antenna transmission scheme of the UE has to be taken into account. When designing the multiple antenna transmission scheme, a sum of outputs of signals transmitted through the multiple antennas of the UE has to be equal to a maximum output considered in single antenna transmission. For example, assume that the maximum output of the single antenna is 23 dBm. Then, if the UE uses two antennas, each antenna may be implemented to have a maximum output of 20 dBm so that a sum of maximum outputs of signals transmitted from the two antennas is 23 dBm. If the UE uses four antennas, each antenna may be implemented to have a maximum output of 17 dBm so that a sum of maximum outputs of signals transmitted from the four antennas is 23 dBm. As such, a power amplifier of a UE using multiple antennas may be designed to have a lower maximum output than a UE using a single antenna. For example, when a power amplifier of a UE having a single antenna has a maximum output of 23 dBm, power amplifiers of respective antennas of a UE having two antennas may be designed to have a maximum output of 20 dBm, and power amplifiers of respective antennas of a UE having four antennas may be designed to have a maximum output of 17 dBm.
A UE using a plurality of power amplifiers in the multiple antenna system may transmit orthogonal reference signals for each Tx antenna. In this case, Tx power of a reference signal may be different from that of a data signal due to the limited maximum output of the power amplifier of the UE, and thus the coverage of the reference signal may not coincide with the coverage of the data signal.
Accordingly, there is a need for a method capable of effectively transmitting an uplink reference signal from a UE using a plurality of power amplifiers in a multiple antenna system.